Making and Knowing
A minimal edition of BnF Ms Fr 640 in English Translation

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[Stone alum]

Stone alum must be reheated in a pot covered with a tile over a good charcoal fire so that the impurities which might be in it [are] burned with that which does not burn. This is done either in the goldsmith forge surrounded by bricks or in a little oven vented to the fusion fire such that the pot stays red for a quarter of an hour. This is more to purify it than for anything else. It becomes reddish on top and the inside remains white and dries best this way.#

at left middle margin of folio 108v

# Once your stone alum is cooled down, crush it in a mortar, then grind it more finely on marble. And this makes it into very fine wooly dust that gives binding to sand without burning like all other things. [This is] a beautiful invention. Crush it on the mortar to being careful to keep the grindings together because the dust flying can get down your collar or on your face which can have very grave consequences. Put a little each time in the mortar to avoid the dust flying. It is best to grind therefore with the pestle trained on the mortar rather than on the marble where it spreads and on the mortar it gathers [the dust] from all sides and renders it very fine and sweet to the touch.